Cooperative nonlinear optical response of molecular aggregates: Crossover to bulk behavior

Abstract

Equations of motion for the nonlinear optical response of arbitrary-size molecular aggregates are derived. The relative role of intramolecular and intermolecular (two-exciton) nonlinearities and the cooperative enhancement induced by the latter are analyzed. A crossover from ∼${\mathit{N}}^2$ to ∼N scaling of the nonlinear polarizability is predicted as the aggregate dimension becomes comparable to the optical wavelength, N being the aggregate size. The limitations of the local-field approximation are demonstrated.